Paper manufacture is among the most water intensive industries. In the course of the paper making process, at various stages substantial amounts of water and aqueous solutions are added to the cellulosic fibers (inflow stream) and separated there from, respectively (effluent stream). Typically, in the course of the process, a relatively concentrated aqueous slurry of cellulosic material, the so-called “thick stock”, is diluted by addition of water, thereby yielding a relatively diluted aqueous slurry of cellulosic material, the so-called “thin stock”.
As a result of growing concern for the purity of water resources and in response to growing governmental pressures to maintain the quality of these water resources, paper industry has been required to investigate and implement methods for reducing chemical pollutants contained in their effluent water streams. The danger of chemical pollution in water is due to the ability of organic constituents of the effluent streams of paper mills to bind dissolved oxygen contained in the water. This binding, whether by chemical reaction or simple chemical interaction, prevents the utilization of dissolved oxygen by aquatic life. The effect of this binding is commonly referred to as chemical oxygen demand (COD).
It is well known that the higher the COD of the waste water to be treated, the more ineffective, more unreliable and more expensive are these processes.
Because of the importance of maintaining adequate levels of dissolved oxygen in water streams, various governmental agencies have set forth guidelines and test procedures for measuring the COD of effluent streams of paper mills entering rivers and lakes. Various processes have been implemented to improve the quality of the water discharged. Among the methods proposed are (1) evaporation followed by incineration, (2) chemical treatment to render the organic constituents in the effluent harmless, (3) biological treatment and aeration of effluent collected in holding tanks, and (4) oxidation of the chemical constituents under restrictive conditions.
WO 01/36740 discloses papermaking processes using enzyme and polymer compositions. The polymer compositions typically contain starch, i.e. fresh starch is added to the system. The reference is fully silent on recycling of starch originating from waste paper. A biocide may be added to the pulp or treated pulp. For example, a biocide may be added to the treated pulp in a blend chest after the pulp has been treated with the enzyme and cationic polymer. The teaching of the reference is focused on utilization of enzymes. It is well known that some biocides interfere with enzymes. The reference does not require the presence of biocide, but merely discloses this as an option to be used in conventional ways for papermaking. There is no hint in the reference that starch degradation can be prevented by addition of biocide, let alone that the thus non-degraded starch can be refixated to the cellulose fibers by means of ionic polymers.
From EP 0 361 736 compositions containing a starch and a flocculating agent intended for use in a paper- or boardmaking furnish are known. US 2006/289139 discloses a method of improving retention and drainage in a papermaking process. The method provides for the addition of an associative polymer, starch or a starch derivative and optionally a siliceous material to the papermaking slurry.
These processes, however, are not satisfactory in every respect and thus, there is a demand
for a method for manufacturing paper, paperboard or cardboard which reduces the COD of the waste water that is produced at the individual stages of the paper manufacturing process including the early stages.
Starch, particularly non-ionic, anionic, cationic and/or native starch, that is released in the wet end of a papermaking machine by the pulping of waste paper or broke is not fixed to fiber except through natural retention and it does not usually contribute to strength parameters. Further, degradation of the starch usually through microbiological activity causes an increase in biological oxygen demand (BOD) and electrical conductivity and a drop in pH due to the creation of organic acids in the papermaking machine system. This leads to deposition, increased need for microbiological control programs, higher uses of new internal or surface starch to reach strength targets and even up to reduced machine productivity. BOD contributes to COD and gives problems in reaching consent targets from the effluent plant.
For production of woodfree uncoated and coated fine papers up to 40 kg starch per ton of paper are applied. Packaging paper made from 100% recovered paper can only be produced economically and in the required quality by adding cost effective biosynthetic starch products. Therefore, these papers are produced with an average starch consumption of 40 kg t−1, mainly by surface application. A further 25 kg t−1 is applied as an adhesive in the converting plant. This means that a high amount of starch is typically returned to the production process via recovered papers, where conventionally it is nearly not retained in the paper sheet. Therefore, this uncontrolled starch quantity leads to a considerable load in the white water circuit (usual COD levels from 5,000 to 30,000 mg O2 l−1) and finally also in the waste water (cf. H Holik, Handbook of paper and board, Wiley-VCH Verlag GmbH & Co. KGaA, 1st ed, 2006, Chapter 3.4.3).
Thus, there is a demand for a method for manufacturing paper, paperboard or cardboard which overcomes these drawbacks of the prior art.